Physical Geology 101 3 Plate Tectonics I p 37 46 Plates in Motion The tectonic plates are constantly in motion Where they collide the compression can result in the production of high Where they rip apart may form Despite our observation of such features for a very long time the theory of plate tectonics is nonetheless a fairly recent idea The beginnings of plate tectonic theory were introduced by Alfred Wegener in the year He called his new hypothesis This concept implied that the continents have not always been in their present positions but have drifted and changed positions over long periods of geologic time What observation about the continents led to Wegener s hypothesis It was hypothesized that all the continents had once been joined together as one large supercontinent called At some point in time this supercontinent broke apart and the continents drifted off to their present locations Initially these ideas were highly criticized by geologists who could think of no mechanism by which the granitic continents could possibly force their way through solid basaltic ocean crust It took many decades for geologists to finally realize what the mechanism is that allows continental drift to actually occur In fact plate tectonics is responsible for the mountain ranges ocean basins ocean current circulation patterns climatic variations distributions evolution and extinctions of plants and animals volcanoes and earthquakes and economic mineral locations It is a unifying theory for all topics of geology Evidence for Plate Tectonics Continental Fit One of the most convincing lines of evidence for continental drift and plate tectonics is the way that the continents seem to fit so well together despite being separated by oceans today Example and It must be more than sheer chance that these coastlines match so well But if we look only at the present day coastlines of these continents there seem to be large gaps between them when we bring them back together But sea level fluctuates through time so the shape of the present day coastline can be misleading What depth below sea level marks the continental edges Using the true edges of the continents as a comparison the largest gap between Africa and South America is only 90 km 56 mi 1 Physical Geology 101 Using the true edges of the continents to match up the ancient jigsaw puzzle pieces the components of all of Pangea fit together very well Matching Geology Matching coastlines is one line of evidence but taken alone it is not enough One of the obvious things to look for if two continents were once joined together is a similarity in their geological characteristics i e the rocks We would expect sedimentary rocks to form on both continents at the same time if they were actually stuck together Do we see similar geology across the continents YES or NO What aged rocks match up very well across the join between Africa and South America So before this time Africa and South America must have been linked together There is also very clear evidence for the continuity of geologic structures like mountain ranges from one continent to the next For example a mountain belt in eastern North America called the matches up across Pangea with similar aged mountains in Ireland Great Britain Greenland and Scandinavia called the What continents of the southern hemisphere show a remarkable similarity between rocks that are Carboniferous to Jurassic in age What do we call this combination of joined land masses that existed in the southern hemisphere during this time period Paleoclimate Evidence If the continents were once joined together and there was an ice age during that time we would expect glacial features to match up across the continents We see this in and where there are similar aged thick glacial deposits that match up across the continents when they are put back together What age are these glacial deposits We also see consistency between the ice motion directions across the continents that formed Gondwana Ice can scratch deep grooves called into bedrock that provide evidence of the direction the ice moved When the Gondwana landmasses are fit back together the direction of the grooves is consistent with the ice sheet radiating away from a central high point in southern Africa 2 Physical Geology 101 The Carboniferous glacial deposits in Gondwana suggest that it was situated near the at that time which was the high point for all this ice What were the northern continents of Pangea collectively called Is there is evidence of for Carboniferous glacial deposits in any of the northern hemisphere continents YES or NO This suggests that Laurasia must have been near the during the Carboniferous Fossil Evidence If the continents were once joined together we might also expect them to have had the same types of plants and animals during the time that they were linked In Gondwana the plants and animals evolved very similarly but after Pangea split up plants and animals evolved very differently because they were totally cut off from each other What type of plant fossil from the Carboniferous period is found in rocks in Africa South America India Australia and Antarctica Could the seeds of this plant have been carried across the oceans by the wind YES or NO Why The only way we could have had this same plant in all these different continents is if the continents were all joined together at some point There are also types of animals that are similar between the continents such as a small freshwater aquatic reptile called Where can fossils of this species be found and This species was far too small to have been able to swim across and entire ocean to get from one continent to the next So the continents must have been together The Wandering of the Poles As if the evidence weren t convincing enough already geophysicists in the 1950s discovered something else that seemed to point towards the existence of continental drift They were looking at the record of ancient magnetism in rocks called As a rock forms any magnetic minerals in the rock become aligned parallel to the Earth s magnetic field of the time For example rocks forming today like lava rocks that erupt from a volcano preserve a record of the magnetic field that exists on Earth today 3 Physical Geology 101 Earth s magnetic field is horizontal near the equator and vertical at the magnetic poles The angle between the magnetic field direction and the horizontal at any point is called the At any point on Earth today the